1. Field of the Invention
This invention is concerned with devices, such as mirrors and windows, having controllable reflectivity.
2. Description of the Related Art
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing "smart windows" with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. These approaches are only partially effective, since the window itself is heated and because these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable low voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating.
In prior art attempts to exploit reversible electrodeposition of a metal for light modulation, the deposits obtained on transparent substrates presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and exhibited poor reflectivity and high light absorbance, especially when thick. Such deposits have been investigated for display applications involving reflectance from the background, with white pigments often being added to improve contrast. Warszawski (U.S. Pat. No. 5,056,899), which is concerned with displays, teaches that reversible metal electrodeposition is most appropriate for display applications, since significant disadvantages for transmission devices were given (e.g., the possibility of metal deposition at the counter electrode). Such teachings imply that the application of reversible metal deposition to smart windows must involve light absorption by the finely divided electrodeposited metal, which would result in heating of the device itself and thus the space inside. The low reflectance of this type of deposit would not be appropriate for adjustable mirror applications. The prior art literature also teaches that, for transmission-type devices, reversible metal electrodeposition requires the use of an auxiliary counter electrode reaction; otherwise, metal would plate on the counter electrode as the deposit was de-plated from the working electrode.
Electrolytes described in the prior art literature contain auxiliary redox species (e.g., bromide, iodide, or chloride) that are oxidized (e.g., to bromine, iodine, or chlorine) at the counter electrode during metal deposition under the high drive voltages used. This introduces chemistry-related instabilities during long term operation and leads to deposit self erasure on open circuit via chemical dissolution of the metal deposit, e.g., 2Ag.sup.0 +Br.sub.2 .fwdarw.2AgBr. In most cases, this auxiliary redox process hinders metal deposition at the counter electrode during erasure, introducing a threshold voltage that is desirable for display applications. This auxiliary redox process may represent a significant side reaction even when metal plating/deplating occurs at the counter electrode and a threshold voltage is not observed. See, e.g., Warszawski, columns 3-4 (when copper or nickel were present in the counter electrode paste) and Duchene et al., Electrolytic Display, IEEE Transactions on Electron Devices, Volume ED-26, Number 8, Pages 1243-1245 (August 1979); French Patent No. 2,504,290 (Oct. 22, 1982). High switching voltages of at least 1 V were used for all the electrodeposition devices which have been found in the patent and literature prior art.
A paper by Ziegler et al. (Electrochem. Soc. Proc. Vol. 93-26 (1993), p. 353) describes an investigation for display applications of the reversible electrodeposition of bismuth in aqueous solutions containing a large molar concentration ratio of halide anions to the trivalent bismuth ion. Halide anion oxidation served as the counter electrode reaction with the 1.5 volt write voltage used. The deposits obtained were dark in color and were shown to decrease the reflectance of the ITO surface. Subsequent reports by these authors (Electrochem. Soc. Proc. Vol. 94-31 (1994), p. 23; Solar Energy Mater. Solar Cells 39 (1995), p. 317) indicated that addition of copper ions to the electrolyte was necessary to attain complete deposit erasure. Thus, Ziegler et al. do not teach that a significant improvement in the deposit quality or erasability can be derived from use of a high molar concentration ratio of halide anion to electrodepositable metal ion in the electrolyte. These authors utilized a counter electrode reaction other than metal electrodeposition/dissolution and never obtained a mirror deposit.
Warszawski teaches that the use of a grid counter electrode would give a less uniform deposit since deposition on the transparent working electrode is highly localized in the vicinity of the counter electrode grid lines (a consequence of the very thin film of gel electrolyte used). Warszawski also teaches the use of an aqueous gel electrolyte to minimize sensitivity to atmospheric contaminants and to avoid the necessity of having a leak tight seal. Such electrolytes, however, have much more limited temperature and voltage operating ranges compared with organic-based electrolytes with high boiling solvents.
One effort to improve the deposit quality of the electrolytic solution used in a reversible electrodeposition process, described in U.S. Pat. No. 5,764,401 to Udaka et al.), requires the addition of organic additives to the solution. Unfortunately, such additives are typically destroyed during the electroplating process, greatly limiting cycle life. Furthermore, this approach fails to produce highly-reflective mirror-like deposits, which, due to their superior heat-rejecting characteristics, are desirable for many applications.
U.S. Pat. No. 5,880,872 to Udaka teaches that the "working" electrode of a reversible electrodeposition structure is degraded, and its working life thereby shortened, by the high voltage required to dissolve the metal film deposited upon it. Udaka states that this consequence can be avoided by adding an alkali metal halide to the device's electrolytic solution, preferably in an amount which provides an alkali metal halide to silver halide ratio of between 0.5 to 5. However, the described electrolytic formulation fails to provide the inherent stability, high quality deposits, good erasure and long cycle life needed for practical applications. Mirror deposits were never obtained.
Prior art literature teaches that the memory effect is temporary. This is a consequence of the occurrence of a counter electrode reaction other than metal plating/deplating. The energetic oxidation products generated at the counter electrode can cause dissolution of the metal deposit on the working electrode either chemically on open circuit (slow) or electrochemically during short circuit (fast).
Nishikitani et al. (European Patent No. 0,618,477) teaches that the counter electrode in electrochromic devices for smart window applications can be a metal grid which is substantially transparent. Since no metal electrodeposition occurs in electrochromic devices, however, the grid in this case is used to provide a transparent electrode, not to maintain transparency by localizing metal deposition. In addition, to provide adequate electrical capacity for electrochromic devices, Nishikitani's grid would need a very high surface area (at least 10 m.sup.2 /g and preferably 50 to 5,000 m.sup.2 /g) and a line width of 50 to 5,000 .mu.m; alternatively, a plurality of dots on a conducting substrate can be used, but the dots must contain fine particles having electrical capacitance of not less than 1 farad/g.